Many manufacturing processes utilize high-purity chemicals entrained in a carrier gas for such processes as semiconductor doping, vapor depostion, etching, and molecular impregnation of a substrate with the entrained chemical. In many of these applications, the purity of the chemical is critical, and impurities are measured in parts per billion. Contamination, such as that which may occur during shipping and handling, must be avoided at all costs.
For example, some of the chemicals used in the manufacture of semiconductor devices are liquid phosphorous oxychloride, phosphorous trichloride, phosphorus tribromide, trichloroethylene, tetramethoxysilane, silicon tetrabromide, trichloroethane, arsenic trichloride, arsenic tribromide, and antimony pentachloride. Many of these chemicals, such as the arsenic compounds, are highly toxic. Others, such as the bromine compounds, are extremely corrosive. Accordingly, worker exposure must be minimized. At the same time, care must be taken to assure that the highest level of purity of these chemicals is maintained. Even the slightest contamination may affect the yield of semiconductor devices, which directly affects the profitability of the overall fabrication process.
In the past, such chemicals have typically been shipped in flame-sealed glass ampules capable of meeting the pertinent Department of Transportation regulation requiring containers capable of holding 15 psi pressure. (Although certain metal containers may also satisfy the applicable DOT regulations, such containers are unacceptable because of the problem of metallic contamination, which is particularly harmful to the reliable manufacture of semiconductors.) When the glass containers were received, they were typically opened by breaking the seal, after which they were emptied into a bubbler. A bubbler is a device which permits a carrier gas, such as nitrogen, to be bubbled through the liquid source material, whereby the liquid source material becomes entrained in the gas. The carrier gas, with entrained chemical, is typically supplied to the substrate to be treated, e.g., in a diffusion furnace or a vapor deposition chamber. As is readily apparent, the transfer of the liquid source material from the glass shipping ampule to the bubbler was a serious potential source of atmospheric and moisture contamination and worker exposure to the chemical.
One relatively satisfactory solution to the contamination and exposure problem is the quartz container disclosed, for example, in U.S. Pat. Nos. 4,134,514, 4,140,735, and 4,298,037. These patents disclose high-purity quartz bubblers which double as shipping containers. The quartz bubblers are filled with chemical by the supplier; the fill tube is flame-sealed, in accordance with DOT regulations; the bubbler containing chemical is then shipped to the user, who attaches gas lines, breaks a seal, and monitors the temperature control equipment to the bubbler, and uses the chemical as desired. Although the majority of contamination problems are thus avoided, since there is no need to transfer the chemical from the shipping container into a separate bubbler, one drawback of this system is the expense involved. High purity quartz containers are relatively costly. For this reason, it has been the practice in the industry to return empty quartz bubblers to the chemical supplier to be refilled. This involves a return shipping expense. Moreover, as the inlet, outlet, and fill tubes are repeatedly heated and resealed, the crystalline structure of the quartz can be affected, causing the quartz to become brittle or crumble. As a result, the bubbler must be carefully examined at each refill time. Some bubblers can only be used as few as three times, others may last for 10 to 12 refills, with the average being in the neighborhood of five to six times.
No suitable less costly alternatives to the quartz bubbler has been apparent to those of ordinary skill in the semiconductor and related supporting industries. Most alternatives considered have been ruled out for failure to satisfy shipping regulations, imcompatibility with the chemicals to be shipped, or contamination of those chemicals by the material itself.
A major problem that appears to rule out the use of organic polymer materials for bubblers is air and moisture contamination of the contained liquid source material. Even minute amounts of moisture contamination can have extremely deleterious effects on semiconductor yield. Although many organic polymers are commonly believed to be impermeable vapor barriers, in truth, small amounts of air and moisture are able to infiltrate nearly all such materials. One graphic illustration of this phenomenon is the gradual shrinking of a child's balloon as pressurized air escapes through the walls of the balloon itself.
The current cost of even small (500 ml.) high-purity quartz bubblers is on the order of several hundred dollars--often approaching or being more than the cost of the chemicals they contain. It is significant that, despite the existence of the problem for a number of years, and the almost overwhelming economic incentive involved, no suitable low-cost alternative bubbler has heretofore been developed.
Accordingly it is an object of the present invention to provide a relatively inexpensive bubbler suitable for transporting toxic and corrosive ultra high-purity liquids. It is a further object of the present invention to provide a disposable bubbler made of organic polymer material. Another object of the present invention is to provide a low-cost bubbler that avoids atmospheric contamination, moisture contamination, and contamination of the contained liquid source material by the container itself. Still another object of the present invention is to provide a valve for use on a bubbler that is vapor impermeable, and yet provides no possibility of metallic contamination of the liquid source material.